Conventionally, techniques for generating power from ambient vibration have been actively developed. Among them, a technique for generating power from piezoelectric elements and a technique for generating power from a change in a magnetic flux density of a permanent magnet are known.
Many of the methods for generating power using piezoelectric elements are performed by generating power by deforming the piezoelectric elements by external force in some way or other. Methods for deforming piezoelectric elements include a method for deforming piezoelectric elements by application of vibration to piezoelectric elements, a method for indirectly applying pressure such as wind pressure or sound pressure, a method for causing an object such as a weight to collide with piezoelectric elements, and a method for attaching piezoelectric elements to a deformed object (for example, refer to Patent Literature 1). Patent Literature 1 discloses a sound power generation apparatus which generates power by a piezoelectric element using air pressure fluctuation caused by sound and a vibration power generation apparatus which generates power by piezoelectric elements using pressure fluctuation caused by vibration.
Moreover, a method for generating power using a change in magnetic flux of a permanent magnet is a method for generating power by a temporal change in interlinkage magnetic flux density of coil caused by vibration of the permanent magnet, that is, a method for generating power using electromagnetic induction (for example, refer to Non Patent Literature 1 and Patent Literature 2).
In Non Patent Literature, a permanent magnet is vibrated within the coil in parallel with a magnetization direction. As a result, a magnetic flux density within the coil is changed which causes a current to flow through the coil. Non Patent Literature 1 discloses a power generating element that generates power using the above phenomenon.
Patent Literature 2 discloses a power generating element including: a bias magnet which is magnetized in two poles; a magnetostrictive material which changes magnetic permeability through an inverse magnetostrictive effect by applying force from outside and changes a flow of magnetic flux; a compressing means which periodically compresses the magnetostrictive material in a direction having magnetic anisotropy; and a coil means which induces current by the periodically changing magnetic flux. In the power generating element, the magnetostrictive material, the coil, and the compressing means are disposed such that the periodically changing magnetic flux and the coil wound around the coil center form a linkage. In other words, this is a structure which generates power with current which is generated in the coil by periodically compressing, in a longitudinal direction, the magnetostrictive material having magnetic anisotropy in a longitudinal direction.
The piezoelectric element disclosed in Patent Literature 1 has large piezoelectric longitudinal constant and high power generation efficiency of piezoelectric vertical effect (when the direction of force and the direction of taking out voltage are the same). However, when power is generated using bending deformation through deforming a single-plate piezoelectric material, voltage is taken out in a direction perpendicular to a direction of force (piezoelectric horizontal effect), with the result that the power generation efficiency is low. Moreover, the piezoelectric material is a brittle material which is easily damaged by bending and impact. Therefore, there is a problem that an excessive load cannot be applied to the piezoelectric material and it is difficult to apply large bending to and have a large impact on the material for increasing power generation capacity. Moreover, the piezoelectric element has high impedance in low frequency since it is electrically inductive load. Furthermore, when a load having lower impedance than the piezoelectric element is connected to the piezoelectric element, voltage generated at the load is low. As a result, there is a demerit that power obtained from power generation is low and power generation efficiency is low.
Moreover, in the method for generating power using a change in linkage magnetic flux density in the coil caused by the vibration of the permanent magnet as disclosed in Non Patent Literature 1, it is necessary to cause a vibrator to vibrate at large amplitude and high frequency for increasing power generation capacity. When the size of the permanent magnet used as the vibrator is large, the mass of the vibrator is high while the resonance frequency of the vibrator is low. As a result, there is a problem that the power generation capacity is not increased.
Moreover, the method for generating power by periodically compressing the magnetostrictive material disclosed in Patent Literature 2 requires large force for compressing the magnetostrictive material in a longitudinal direction. Moreover, there is a problem that since the compression force is unevenly applied to the magnetostrictive material, power generation efficiency is low.